1. Field of the Invention
This invention relates to a knee prosthesis for fitting to a patient as a replacement knee joint.
2. Prior Art
Modern total knee replacement involves the resurfacing of the femoral condyles with a metallic component, roughly approximating to the shape of the anatomical femoral condyles, and resurfacing the tibial plateau with a polyethylene component having a metallic base plate. Conformity between the polyethylene of the tibial component and the metallic femoral component has historically been a troublesome area. Ideally the femoral component should be congruent with the top of the tibial component in order to minimise polyethylene wear. The difficulty, however, is that the knee joint does not act as a fixed axis hinge. During normal movements of the knee, rotation of the femur upon the tibia occurs, and roll back of the femoral condyles upon the tibia occurs, particularly when the knee is flexed.
UK Patent No 1,534,263 discloses a knee joint device formed by a femoral component, a floating plastics material meniscal component, and a flat topped tibial component. This device allowed congruency between the femoral and meniscal components, but since the meniscal component was free to move antero-posteriorly and to rotate, rotational and sheer stresses were not generated at the tibial implant fixation interface and loosening did not occur. Moreover wear of the polyethylene was found to be low. However this device suffered from the disadvantage that under certain circumstances posterior or medio lateral dislocation could occur, with the meniscal component possibly being displaced into the synovium.
Whilst the device of UK Patent No 1,534,263 was generally concerned with a replacement joint to address unicompartmental disease (medial arthritis of the knee), where the cruciate ligaments and lateral compartment of the knee were left untouched, it is common practice to perform a total knee replacement, where the lateral component of the knee is also resurfaced and the anterior cruciate ligament or both cruciate ligaments are resected. A commonly employed total knee replacement is the Insall Burstein replacement in which both cruciate ligaments are resected and a posterior stabilised design of knee replacement is inserted. The provision of a bearing in the form of a cam mechanism between the femoral component and the polyethylene tibial component means that with increased flexion of the knee increased posterior translation of the femoral component upon the tibia occurs, partially simulating the normal kinematics of the knee. The cam mechanism replaces the function of the cruciate ligaments. However although the kinematics in terms of femoral roll back are adequately addressed, the bearing between the tibial and femoral components is incongruent, and therefore theoretically undesirable, resulting in high contact stress, leading to plastics wear.
Another type of total knee replacement involves retention of the posterior cruciate ligament, with a polyethylene meniscus, broadly congruent with the femoral component, being allowed to rotate and/or slide on a polished metal tibial plateau, thereby addressing both the issue of high contact stress and also tibial loosening. A disadvantage of this type of knee replacement relates to the function of the posterior cruciate ligament, with the anterior cruciate ligament resected. For the cruciate ligaments to act effectively, both the anterior and the posterior ligaments do have to be present in the knee, as they represent a four bar linkage between the femur and the tibia. It has been shown that isolated retention of the posterior cruciate ligament does not adequately allow the normal kinematics of the knee to occur. Accordingly upon flexion of the knee, instead of the normal posterior translation of the femur upon the tibia, the reverse is often found to occur, whereby the femoral component abnormally translates forwards on the tibia, with, in the worst case, posterior dislocation.
This has two undesirable side effects. Firstly the patello-femoral contact pressures are abnormally increased, thus increasing the potential for pain at the front of the knee, or loosening of the patellar component, or fracture of the patella. Secondly a patient with this abnormal anterior translation may not be capable of full flexion of the knee after replacement, as soft tissue impingement between the femur and the tibia posteriorly can occur when the normal femoral roll back in flexion does not happen.